The origin of the elements remains one of the largest unanswered questions in nuclear physics today. In particular, nucleosynthesis processes involving nuclei far from stability (i.e., the rp- and r-processes) remain largely untested. This award will fund the participation of this group in an initial experiment that has been approved for a direct mass measurement of proton drip-line nuclei relevant to the astrophysical rp-process. This work involves an international collaboration in nuclear astrophysics at the RIKEN Radioactive Ion Beam Factory (RIBF). Future experiments are planned to directly measure the masses of neutron-rich nuclei relevant to the r-process of nucleosynthesis, as well as experiments to measure nuclear structure far from stability. Experiments involving very exotic nuclei are made possible by the high intensities of the RIBF.
As broader impacts of the program, both graduate and undergraduate students will participate in the program, providing exposure to science at an international level. An international collaboration will be engaged in the proposed work, and an American presence will be established at what is currently the most powerful heavy ion accelerator on the planet.
"We are made of star stuff." It is a fascinating fact of nature that the same heavy elements in the oldest stars in the galaxy came from the same process that formed the heavy elements on our planet and in our bodies. Because of this, the National Academy of Science has reported that the origin of the elements remains one of the largest unanswered questions in physics today. While production of the light elements is explained by ordinary fusion processes in stars, the production of elements heavier than iron is postulated to occur via more exotic, explosive nuclear processes. However, the question as to how these elements such as gold, many common metals, and others form remains to be answered completely. This process is thought to occur in a very hot, explosive, neutron-rich environment in which heavy elements are formed from exotic, unstable elements, which are produced in a series of extreme nuclear reactions. The site of this process is thought to be supernovae explosions and is responsible for nearly 70% of all nuclei heavier than iron and nearly all of the rare-earth elements, which are used in many household devices, such as cellular phones, household electronics, and automobiles. It is important to note that the production of these elements in nature is closely related to the properties of the atomic nuclei involved. There is then a very close relationship between the supernova environment and the nuclear properties of elements formed in supernova explosions. Thus knowledge of nuclear properties relevant to these elements and their creation (known as "nucleosynthesis") will provide a better basis for analyzing data collected by current and future astronomical observations to better constrain the origin of the elements in our universe. This work addresses questions which provide insight into astronomical observational data as well as nuclear physics properties relevant to these data. The process whereby heavy elements are formed in supernova explosions is known as the r-process (short for "rapid-process" for its speed). Recently, astronomical observations have indicated that this r-process may be much more complex than we thought in that it may have a well-understood "strong" component and a more mysterious "weak" component. Collaborators in Japan and the US are working to understand a possible environment of the weak component. Two simultaneous and closely connected opportunities exist for the pursuit of studies in this field. The first opportunity is the development of a theoretical framework to better understand how the heavy elements are produced and to better reconcile astronomical observations with what is known about the nuclear structure of the elements involved in supernova explosions. The second is the development of an experimental program at the Radioactive Ion Beam Factory (RIBF) in Wako, Saitama, Japan. The RIBF is currently one of the most powerful nuclear physics facilities of its kind in the world and currently one of the only facilities capable of producing exotic atomic nuclei in quantities significant enough for the study of supernova nucleosynthesis. I’ve already received approval to begin this program to study the production of exotic elements relevant to the structure and production of heavier elements. Data collected in the experimental portion at the RIBF will be used in the theoretical calculations; thus the two program parts are closely related. US scientists continue to work with astronomers and physicists at the National Astronomical Observatory and the RIBF to accomplish these goals.
